CURRENT

CURRENT

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A current is any motion of charge from one region to another. In this seetion e’ll discuss currents in conducting materials. The vast majority of technological applications of charges in motion involve currents of this kind. In electrostatic situations (discussed in Chapters 22 through 25) the electric field is zero everywhere within the conductor, and there is no current. However, this does not mean that all charges within the conductor are at rest. In an ordinary metal such as copper or aluminum, some of the electrons are free to move within the conducting material. These free electrons move randomly in all directions, somewhat like the molecules of a gas but with much greater speeds, of the order of 106 mis. The electrons nonetheless do not escape from the conducting material, because they are attracted to the positive ions of the material. The motion of the electrons is random, so there is no net flow of charge in any direction and hence no current.

Now consider what happens if a constant, steady electric field E is established a conductor. (We’ll see later how this can be done.) A charged particle (such as electron) inside the conducting material is then subjected to a steady force i-qE. charged particle were moving in vacuum, this steady force would cause a in the direction of i,and after a time the charged particle would be that direction at high speed. But a charged particle moving in a conductor under collisions with the massive, nearly stationary ions of the material. In each  collision the particle’s direction of motion undergoes a random change. The net the electric field E is that in addition to the random motion of the charged p within the conductor, there is also a very slow net motion or drift of the moving c particles as a group in the direction of the electric force i-qE (Fig. 26-1). This is described in terms of the drift velocity Vd of the particles.

As a result, there is current in the conductor. As mentioned above, the random motion has a very large age speed; by contrast, the drift speed is very slow, often of the order of 10-4 mls. The drift of moving charges through a conductor can be interpreted in terms of – and energy. The electric field E does work on the moving charges. The resulting energy is transferred to the material of the conductor by means of collisions wi  ions, which vibrate about their equilibrium positions in the crystalline structure conductor. This energy transfer increases the average vibration energy of the io therefore the temperature of the material. Thus much of the work done by the e field goes into heating the conductor, not into making the moving charges movefaster and faster. This heating is sometimes useful, as in an electric toaster, but in situations is simply an unavoidable by-product of current flow.

In different current-carrying materials the charges of the moving particles positive or negative. In metals the moving charges are always (negative) electrons. in an ionized gas (plasma) or an ionic solution the moving charges include both and positively charged ions. In a semiconductor material such as germanium or conduction is partly by electrons and partly by motion of vacancies, also kn holes; these are sites of missing electrons and act like positive charges.Figure 26-2 shows segments of two different current-carrying materials. In =-  26-2a the moving charges are positive, the electric force is in the same directionand the drift velocity Vd is from left to right. In Fig. 26-2b the charges are negate electric force is opposite to E, and the drift velocity vd is from right to left. In either  there is a net flow of positive charge from left to right, and positive charges end up .right of negative ones. We define the current, denoted by /, to be in the direction in there is a flow of positive charge. Thus we describe currents as though they co entirely of positive charge flow, even in cases in which we know that the actual is due to electrons. Hence the current is to the right in both Fig. 26-2a and Fig.'” –  This choice or convention for the direction of current flow is called convention rent. While the direction of the conventional current is not necessarily the same direction in which charged particles are actually moving, we’ll find that the sign moving charges is of little importance in analyzing electric circuits.

Figure 26-3 shows a segment of a conductor in which a current is flowing. ‘e seeder the moving charges to be positive, so they are moving in the same direction current. We define the current through the cross-section area A to be the net charge ing through the area per unit time. Thus if a net charge dQ flows through an time dt, the current I through the area along the length of the wire, regardless of whether the wire is straight or curved.

No single vector could describe motion along a curved path, which is why current is not a vector. We’ll usually describe the direction of current either in words (as in “the current flows clockwise around the circuit”) or by choosing a current to be positive if it flows in one direction along a conductor and negative if it flows in the other direction ..•• The SI unit of current is the ampere; one ampere is defined to be one coulomb per second (1 A = 1 Cis). This unit is named in honor of the French scientist Andre Marie Ampere (1775-1836). When an ordinary flashlight (Dv cell size) is turned on, the curl tD the flashlight is about 0.5 to 1 A; the current in the wires of a starter motor used 10 a car engine is around 200 A. Currents in radio and television circuits are usually expressed in  (I mA = lO-JA) or micro amperes (1 J.l.A= 1O~A), and D computer circuits are expressed in nano amperes (1 nA = 10-9 A) or picoamperes = 10-12 A).

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